26 Br. C. B. Walcott's Cambrian 



3. Elliptocephala Stage. The first fourteen segments are of the 

 uniform type, while the posterior five segments are short and have 

 long median spines. 



4. Holmia Stage. The sixteen segments of the thorax are all of 

 the uniform type. 



5. Paedeuniias Stage. The third segment is enlarged and the 

 fifteenth segment is developed into a long spine. Beneath and 

 behind this spine there are from two to six similar but smaller 

 spines. 



6. Olenellus Stage. There are only fourteen segments, of which 

 the third is enlarged ; the fifteenth segment has developed into 

 a strong telson. 



These changes in the thorax are sufficiently marked to form good 

 generic delimitations in most families of trilobites, hut it seems 

 possible that the Mesonacidse were undergoing rapid evolution ; thus 

 Olenellus thompsoni goes through a Holmia and Paedeumias stage 

 before reaching the true Olenellus stage, and it may be that the 

 discovery of new material will produce adults of transition stages, 

 which will make generic identification difficult where genera have 

 been made somewhat lavishly. It cannot be assumed that the 

 collections of these Lower Cambrian forms are within measurable 

 distance of completion. 



A tentative sub-zoning of the Lower Cambrian is suggested by 

 Walcott, based partly on the known stratigraphical occurrences of 

 the Mesonacidce in the few rare instances where successive forms 

 exist in the same region, but mainly on the order of development. 

 D. Olenellus Zone (Upper). 

 C. Callavia Zone. 

 B. Elliptocephala Zone. 

 A. Neva&ia Zone (Lower). 

 It will be interesting to find how far this tentative scheme will stand 

 the test of future work. In Europe there are at present no positive 

 facts by which the merits of the classification can be tested, but the 

 series of beds of the Solva type of lithology, which occur in Wales 

 and at Nuneaton in the English Midlands, are suggested as a hunting- 

 ground — we cannot call it a "happy" one — which ought to be 

 exhausted. 



Another subject of general interest is the sudden appearance of life 

 in the Cambrian period, which has for long engaged the attention of 

 geologists, and acted as a harmless safety-valve when the impetus to 

 theorize would not be denied. But in the case of the papers under 

 discussion, behind the explanation there is a unique knowledge of 

 the stratigraphical relationships of the Cambrian and pre-Cambrian 

 rocks in North America, coupled with the new light shed on Cambrian 

 stratigraphy by the recent researches of Mr. Bailey Willis and 

 Mr. Blackwelder in China and of H.M. J. Deprat and H. Mansay in 

 Yun-nan. The suggestion that there is an extensive break in the 

 succession between the Cambrian and pre-Cambrian in all known 

 localities, and that the Algonkian deposits are all epicontinental, 

 appears to be the most probable explanation of the sudden appearance 

 of prolific organic life in the Lower Cambrian. However, there 



